I find it hard to believe that this is still not implemented. The other questions in QuickBooks Community I have found referencing this are clearly asking for a SSO implementation to log into QuickBooks Online. Yet responses from Intuit are directing people to the Intuit developers page for creating their own custom app using Intuit for SSO.

Please Intuit, there must be someone working in identity that understands SAML that could respond to this request. We just want to be able to leverage our own IdP via SAML to log into Quickbooks Online. Kind of how you can currently "Sign in with Google", except using an IdP that is not Google (Azure, Okta, generic SAML, etc).